1. Field of the Invention
This invention relates to a lens barrel detachably attached to a camera body, in which either an auto-focusing (AF) mode or a manual-focusing (MF) mode is selectable.
2. Description of the Related Art
Such a lens barrel comprises an immovable cylindrical member adapted to be detachably attached to a camera body, and a movable cylindrical member concentrically disposed in the immovable cylindrical member. The movable cylindrical member serves as a lens holder for holding a group of focusing lenses, and is movable along an optical axis of the focusing lenses to perform a focusing adjustment in accordance with a distance between the camera and an object to be photographed.
A drive mechanism is provided between the lens holder and the immovable cylindrical member to move the lens holder along the optical axis, enabling the focusing adjustment to be performed, and is operatively joined to a drive motor, provided in the camera body, upon attaching the lens barrel to the camera body. The drive motor is driven in accordance with an auto-focusing system incorporated in the camera, and a rotational movement of the drive motor is converted into a translational movement by the drive mechanism, which is then transmitted to the lens holder, such that the focusing adjustment is performed.
On the other hand, the lens barrel is provided with a manual-focusing ring rotatably provided around the immovable cylindrical member, and the manual-focusing ring is engaged with the drive mechanism. Thus, a manual rotational movement of the manual-focusing ring is also converted into a translational movement, which is then transmitted to the lens holder, such that the focusing adjustment is performed.
Of course, the lens barrel is further provided with a focusing-mode changeover mechanism for changing from the AF mode to the MF mode and vice versa.
When the AF mode is selected by the focusing-mode changeover mechanism, the drive mechanism is only actuated by the drive motor provided in the camera body, whereby an auto-focusing adjustment is carried out. In this case, of course, the drive mechanism is disengaged from the manual-focusing ring such that the transmission of the movement of the manual-focusing ring to the lens holder is prohibited.
When the MF mode is selected by the focusing-mode changeover mechanism, the drive mechanism is driven by only the manual-focusing ring, whereby a manual-focusing adjustment is carried out. Of course, in the MF mode, the drive mechanism is disconnected from the drive motor such that the transmission of the movement of the drive motor to the lens older is prohibited.
Unexamined Japanese Patent publication No. S63-193119 discloses a type of lens barrel, in which an AF/MF changeover-operating ring for manually actuating the focusing-mode changeover mechanism is movably mounted on the immovable cylindrical member. Either the AF mode or the MF mode is selected by manually operating the AF/MF changeover-operating ring. This conventional type of lens barrel is relatively bulky because both the manual-focusing ring and the changeover ring are mounted on the immovable cylindrical member. Also, a camera using this lens barrel is troublesome to manipulate, because it is necessary to independently operate the manual-focusing ring and the AF/MF changeover-operating ring.
Unexamined Japanese Patent publication No. H04-344609 discloses another type of lens barrel having a single ring, which serves both as a manual-focusing ring and an AF/MF changeover-operating ring. In this type of lens barrel, the single ring is rotatably mounted on the immovable cylindrical member, but it is movable along a longitudinal axis of the immovable cylindrical member between an auto-focusing (AF) position and a manual-focusing (MF) position.
When the single ring is at the AF position, the AF mode is selected. In this case, the drive mechanism is only actuated by the drive motor provided in the camera body, whereby an auto-focusing adjustment is carried out. Namely, the drive mechanism is disengaged from the manual-focusing ring such that the transmission of the movement of the manual-focusing ring to the lens holder is prohibited.
On the other hand, when the single ring is moved from the AF position to the MF position, the MF mode is selected, i.e. the single ring serves as the manual-focusing ring. In this case, the drive mechanism is disconnected from the drive motor, and a resiliently-frictional engagement is established between the drive mechanism and the single ring or manual-focusing ring. Thus, a manual rotational movement of the single ring or manual-focusing ring is converted into a translational movement by the drive mechanism, which is then transmitted to the lens holder, such that the focusing adjustment is performed.
Nevertheless, the type of lens barrel, as disclosed in Unexamined Japanese Patent publication No. H04-344609, is unreliable, because a slippage in the resiliently-frictional engagement may occur between the drive mechanism and the single ring or manual-focusing ring. The severity of the slippage increases as the lens barrel ages, due to wearing of parts undertaking the resiliently-frictional engagement.
To ensure a proper resiliently-frictional engagement between the drive mechanism and the single ring or manual-focusing ring over a long service life period, it is possible to incorporate a highly resilient spring in the drive mechanism. Nevertheless, this approach is unadvisable, because a distortion or deformation may occur in the lens barrel due to the high resiliency of the spring.